1. Field of the Invention
The present invention generally relates to an exposure apparatus, and more particularly to an exposure apparatus used to exposure a substrate to be processed, such as a single crystalline substrate for a-semiconductor wafer. The present invention is suitable to, for example, an exposure apparatus for exposing a single crystalline substrate for a semiconductor wafer by a step-and-scan method in a photolithographic process.
2. Related Background Art
Up to now, there has been used a projection exposure apparatus for projecting a circuit pattern drawn on a reticle or a mask (these terms are interchangeably used-in the present application) to a wafer or the like by a projection optical system to transfer the circuit pattern when a minute semiconductor device such as a semiconductor memory or a logic circuit is produced using a photolithographic (printing) technique.
Recently, as shown in FIGS. 9 and 10, in order to improve a resolution and to expand an exposure region, a scanning projection exposure apparatus 1000 that exposes the entire reticle pattern to each exposure region of a wafer WP through a projection optical system 1200 by illuminating a portion of a reticle RC and scanning the reticle RC and the wafer WP in synchronous with each other using a reticle stage 1100 and a wafer stage 1300 (which is also called a “scanner”) attracts lots of attention. FIG. 9 is a schematic perspective view showing an exemplary structure of a conventional scanning projection exposure apparatus 1000. FIG. 10 is a schematic cross sectional view showing the exemplary structure of the conventional scanning projection exposure apparatus 1000. Note that an illumination apparatus that illuminates the reticle RC on which a circuit pattern is formed is omitted in FIGS. 9 and 10.
A reticle side reference plate (hereinafter referred to as an “R side reference plate”) 1110 which a plurality of positional measurement marks are provided on a reflective surface is fixedly disposed in a predetermined area near the reticle RC on the reticle stage 1100. A wafer side reference plate (hereinafter referred to as a “W side reference plate”) 1310 where a plurality of positional measurement marks are provided on a reflective surface is fixed disposed in a predetermined area near the wafer WP on the wafer stage 1300. In the scanning projection exposure apparatus 1000, a focus detection system 1400 is provided as a focal position detecting unit that detects the positional measurement marks and measures a displacement in position of the wafer WP in the optical axis direction of the projection optical system 1200.
However, when the projection optical system 1200 absorbs exposure heat or when a surrounding environment varies, an error is caused between a measurement origin of the focus detection system 1400 and the focal plane of the projection optical system 1200. Therefore, in order to measure the error for the correction (focus calibration), a through-the-reticle (TTR) alignment optical system 1500 is constructed.
The TTR alignment optical system 1500 has a function of a position detecting unit that detects relative positions between the R side reference plate 1110 and the W side reference plate 1310. The detected relative positions are used to calculate a base line of an off-axis alignment optical system 1600 and a deviation between the scanning direction of the reticle stage 1100 and the scanning direction of the wafer stage 1300 (XY calibration). Here, the base line indicates a distance between a shot center at the time of wafer alignment and a shot center at the time of exposure (optical axis of the projection optical system). In some cases, a value of the base line changes according to various environmental factors or the like. Therefore, it is necessary for the exposure apparatus to measure the base line and to correct it. Note that, as shown in FIG. 11, the TTR alignment optical system 1500 can be constructed so as to detect the relative positions between the R side reference plate 1110 and the W side reference plate 1310 from, the rear side of the W side reference plate 1310, that is, the image plane side of the projection optical system 1200. FIG. 11 is a schematic cross sectional view showing an exemplary structure of the conventional scanning projection exposure apparatus 1000.
In the exposure apparatus, a reduction in size of a pattern to be transferred, that is, an increase in resolution is increasingly required according to an increase in scale of integration of the semiconductor devices. It is impossible to satisfy such requirements only through the reduction in exposure light wavelength. Therefore, in recent years, in order to satisfy the requirement for the increase in resolution, in addition to the reduction in exposure light wavelength, a numerical aperture (NA) of the projection optical system is increased from a conventional NA of about 0.6 to a high NA which exceeds 0.8.
Thus, because the focal depth becomes extremely smaller than conventional ones, a significant improvement in detection precision of the focal position, in particular, an improvement in precision with respect to the focus calibration is required. In addition, alignment precision increases with increasing a resolution, so that a further improvement in precision is required for the XY calibration such as the measurement of the base line or the like.
On the other hand, an improvement in throughput (the number of wafers processed per unit time) is required for the exposure apparatus. Therefore, in addition to the improvements in measurement precision with respect to the focus calibration and the XY calibration, shortening a measurement time is greatly demanded.
In the conventional scanning projection exposure apparatus 1000, when the scanning direction is used as the Y-axis direction, a first detection system 1510 and a second detection system 1520 of the TTR alignment optical system 1500 are generally disposed on an axis parallel to the X-axis within an exposure slit so as to become symmetrical about the Y-axis. Measurement results at two image heights (measurement points) are used for the focus calibration and the XY calibration. In particular, in the case of detecting a focal surface position using the TTR alignment optical system 1500, as shown in FIG. 12, respective drive areas (detection areas) MEa and MEb of the first detection system 1510 and the second detection system 1520 are made symmetrical about the optical axis of the projection optical system 1200 on the X-axis. While the symmetry about the optical axis is maintained, a position of the first detection system 1510 and a position of the second detection system 1520 are changed. Then, the focal measurement is performed at a plurality of image heights (measurement points) KP to define an exposure image plane in a direction perpendicular to the scanning direction (X-axis direction). FIG. 12 is a schematic view showing drive (detection) areas of the conventional TTR alignment optical system 1500.
In the conventional TTR alignment optical system 1500, when the exposure image plane in the X-axis direction is determined, it is necessary to drive the first detection system 1510 and the second detection system 1520. Therefore, measurement takes a long time and a reduction in measurement precision resulting from a drive error is caused.
When the NA of the projection optical system becomes a high NA which exceeds 0.8, the focal depth greatly decreases. Therefore, it is required that, the tilt of the image plane in a direction parallel to the scanning direction (Y-axis direction) in which a problem is not caused up to now is measured, the image plane is corrected by driving, for example, a lens in the projection optical system according to the tilt to be aligned with the actual exposure surface, and exposure is performed. However, as shown in FIG. 12, the TTR alignment optical system 1500 performs the measurement at the plurality of image heights (measurement points) symmetrical about the Y-axis in the exposure slit ES. Thus, the tilt of the image plane in the direction parallel to the scanning direction cannot be measured.
Further, a two-dimensional CCD device is generally used for sensors 1512 and 1522 in the first detection system 1510 and the second detection system 1520 that compose the TTR alignment optical system 1500. Mark measurement is performed by picking up enlarged images of the positional alignment marks formed onto the two-dimensional CCD device. However, the photoelectric conversion efficiency of the two-dimensional CCD device decreases with shortening an exposure light wavelength. Therefore, it is required that intenser light is made incident into the two-dimensional CCD device. When the intenser light is made incident into the two-dimensional CCD device, it is necessary to emit the intenser light from a light source or to reduce a enlargement magnification of the TTR alignment optical system 1500, because the transmittances (efficiencies) of all optical systems such as the TTR alignment optical system 1500 and the projection optical system 1200 drop by shortening the exposure light wavelength.
However, it is technically very hard to emit the intense light from the light source. In addition, even if the intense light can be emitted, a problem regarding the durability of the optical systems is caused. On the other hand, when the enlargement magnification of the TTR alignment optical system 1500 is reduced (for example, when the enlargement magnification is changed to ½, the illuminance on an image pickup device quadruples), a reduction in measurement precision is caused.